1. Field of the Invention
This invention relates to an AM radio receiver a and more particularly, to an AM radio receiver free from tracking errors.
2. Description of the Related Art
There is currently known an AM radio receiver which receives AM broadcasting waves (RF signals) transmitted from broadcasting stations as described in "Bipolar Integrated Circuit for Automobile Audio Receiver" on page 73 of '88 Sanyo Semiconductor Data Book (CQ Publishing Co., Ltd.).
FIG. 1 of the accompanying drawings shows a conventional radio receiver similar to the radio receiver described above. The radio receiver comprises an antenna 10 for receiving RF signals, an antenna damping circuit 12 for attenuating the received RF signals, an RF amplifier 18 including an FET 14 for amplifying the attenuated RF signals and AGC transistor 16 for receiving AGC signals at its base, an RF tuner 20, a mixer MIX 24 for mixing the RF signal from the RF tuner 20 and local oscillator signals from a local oscillator (L. OSC) 22, an IF amplifier (IF) 26 for amplifying IF (intermediate frequency) signals from the mixer 24, and an AM detector 28 for performing AM detection of output signals from the IF amplifier 26.
Such a radio receiver is designed so that the difference between a resonance frequency f.sub.RF of the RF tuner 20 and the resonance frequency f.sub.OSC of the local oscillator 22 are equal to an IF signal frequency (f.sub.IF =450 KHz).
To perform automatic tuning, a tuning signal V.sub.T to be applied to the RF tuner 20 is initially varied in accordance with an inter-station frequency. The inter-station frequency represents a minimum spacing between waves transmitted from respective broadcasting stations. The frequencies used by the respective broadcasting stations should be separated by the predetermined spacing to prevent any interference between the broadcasting waves. Usually, specific broadcasting frequencies are respectively assigned to the broadcasting stations. During automatic tuning, an IF signal for each station internal frequency is detected to seek a broadcasting station having a signal strength greater than a predetermined value.
However, in the conventional radio receiver, an adjustment is made for frequencies which tune only at a few discrete tracking points (e.g. 600 KHz, 1000 KHz, 1400 KHz) in a receiving frequency band. As shown in FIG. 3, at the other receiving frequencies, tuning points of the RF tuner deviate from frequencies of the broadcasting waves determined for every station internal frequency, thereby causing tracking errors.
In the conventional AM radio receiver, gain of the RF amplifier tends to be deteriorated. With an AM stereo radio receiver, separation is sometimes worsened. Further, there has been a problem that automatic tuning cannot be always stopped stably at an optimum point. These problems are remarkable at frequencies (522 KHz to 600 KHz) which are lower than the first tracking point (600 KHz), and at frequencies (1400 KHz to 1602 KHz) which are higher than the third tracking point (1400 KHz).
With the conventional art in which RF tuning is performed by the tuning signal V.sub.T determined according to station internal frequencies, the RF tuning circuit is not always tuned to the maximum point of the signal strength.